Phenolic resins include novolak resins synthesized by reacting phenols with formaldehyde in the presence of an acid catalyst and resol resins synthesized by reacting phenols in the presence of a basic catalyst. It is well known that the former resins give cured articles on heating together with hexamethylenetetramine or the like curing agent, while the latter resins on heating by themselves.
On the other hand, thermosetting resins, like the above-mentioned phenolic resins, can be obtained by reacting modified novolak-type resins of the general formula, ##SPC2##
Wherein R is phenyl, diphenyl, diphenyl ether, diphenyl methane, diphenyl ketone, diphenyl sulfone or naphthalene group or substituted groups thereof, with hexamethylenetetramine or with paraformaldehyde or the like formaldehyde source in the presence of a basic catalyst. Particularly, thermosetting resins obtained by blending modified novolak-type resins of the general formula (I) with hexamethylenetetramine (hereinafter referred to as "H-type resins") are excellent in heat resistance after curing but cannot give sufficiently cured articles unless heated at high temperatures (150.degree. to 200.degree.C) over long periods of time (more than 40 hrs.). Since the H-type resins are so low in curability (or hardenability) as mentioned above, they have such disadvantages that laminates thereof tend to form cracks or cause peeling of layers during after cure. In contrast thereto, thermosetting resins obtained by reacting the aforesaid modified novolak-type resins with paraformaldehyde or the like formaldehyde source in the presence of a basic catalyst (hereinafter referred to as "P-type resins") are far more excellent in curability (curable at about 180.degree.C for 3 hours) than the above-mentioned H-type resins, but are undesirably low in heat resistance of cured articles.